Sensitization remains the central hazard. When austenitic grades dwell between roughly 425 °C and 815 °C, chromium carbides precipitate at grain boundaries, depleting adjacent zones of chromium and creating pathways for intergranular attack. The International Molybdenum Association's practical fabrication guidelines and numerous failure analyses confirm that this risk intensifies with higher heat input and slower cooling. Low-carbon "L" grades (carbon typically ≤0.03 %) reduce the available carbon for carbide formation, yet they alone are insufficient without disciplined process control.
Compliant practice begins with procedure qualification under ASME Section IX for pressure equipment and AWS D1.6 for structural stainless assemblies. Both require documented Welding Procedure Specifications and welder performance qualifications, but chemical service demands additional corrosion validation such as ASTM G48 pitting tests or ASTM A262 intergranular corrosion evaluations. Gas tungsten arc welding with pure argon shielding and mandatory root-side purging keeps oxygen levels low enough to limit heat tint and oxide scale. Heat input is deliberately held below 2.0 kJ/mm for most austenitic grades, interpass temperatures are limited, and stringer beads replace heavy weaving. Post-weld mechanical cleaning followed by nitric- or citric-acid passivation restores the passive layer-an essential step that one Gulf Coast audit found was routinely skipped, with predictable results in chloride environments.
Dissimilar joints between carbon steel and stainless further require over-alloyed fillers such as 309L to accommodate dilution and carbon migration. When these controls are applied consistently, operational data from the reviewed plants show measurably longer intervals between maintenance interventions.
Beyond the pressure boundary itself, secondary structures, supports, and cladding in chemical plants also face aggressive atmospheres. Here, high-corrosion-resistance Al-Zn coated steel and hot-dip galvanized coil offer proven atmospheric and mild chemical durability when properly specified.
Treating every weld as a potential initiation site is not caution for its own sake; it is the practical outcome of years of comparing documented procedures against actual plant performance. Specify low-carbon grades, qualify heat input and purging, restore the passive film, and verify results with corrosion testing. The plants that do so avoid the costly interruptions that still appear when these steps are treated as optional.
Contact Sino-Galvanized for technical support on coated steels suited to industrial and chemical environments, including coating options and export documentation.
